The measurement of permeability is of great importance in several fields which apply the principles of porous-media physics. (1) In hydrology, permeability measurement is important in studies of infiltration, redistribution of water in the unsaturated zone, recharge of aquifers, and groundwater flow within aquifers or across confining layers of aquifers in response to natural or artificially generated pressure gradients. (2) In the petroleum industry, the measurement of permeability of porous rock with oil filling or partly filling its pores is essential in the investigation of oil-bearing rock formations and in the optimum design of oil-extraction systems. (3) In environmental engineering, the permeability of soils, sediments, rocks, clays, and artificial porous media is important in designing and locating toxic waste disposal facilities and in predicting the vulnerability of aquifers to contamination. The objectives here are to isolate the waste to the greatest extent possible, and, if leaks do occur, to minimize the transport of toxic substances to a broader environment. Permeability relates directly to the matter of isolation. It relates indirectly to the transport of dissolved toxic materials, being an essential component of predictive solute-transport models. (4) In agriculture, permeability is important for knowledge of the transport of water in and near the root zone, related to the problems of irrigation and drainage practices as well as of crop and soil management. (5) In soil mechanics, permeability is important as one of the factors that determines the soil water content at a given time and place, which in turn influences the rate of soil consolidation as well as the mechanical properties of the soil related to the question of stability of buildings and other structures.
Previously, the most accurate method of measuring liquid conductivity was by measuring the steady-state flow rate with gravity as the driving force. The weakness of gravitational force means that the measurement takes a long time, prohibitively long when the conductivity is very low. This is the case when pores of the medium are very small (as in fine clays) or, for unsaturated flow, when the liquid of interest occupies only a small fraction of the pore space (as in soils that have been partially drained or dried from a saturated state). Examples of steady-state methods may be seen in the journal Physics, vol. 1, pages 318-333, 1931; and U.S. Pat. No. 2,345,935. Many techniques employing unsteady flow are available for measuring low permeabilities, but the theoretical basis of these methods involves approximations or assumptions due to which the results of the measurements are not very accurate. Examples of these techniques may be seen in Soil Science of America Proceedings 20, pages 458-462, 1956; Soil Science Society of America Proceedings 17, pages 206-209, 1953. The present invention, by permitting substitution of centrifugal force for gravity, reduces the time required for a steady-state measurement, extending the method to finer or less saturated media without a great sacrifice in accuracy. Centrifugation has previously been used in several applications to flow through porous media, but not in connection with steady-state permeability measurements, see for example: U.S. Department of Agriculture Bureau of Soils Bulletin 45, 1907; U.S. Department of Agriculture ARS 41-134, page 8, 1967; Soil Science Society of America Proceedings 3, pages 65-69, 1938; Soil Science Society of America Proceedings 40, pages 212-218. 1976; and AIME Transactions 160, pages 114-123, 1945.